U.S. patent number 6,249,034 [Application Number 09/280,434] was granted by the patent office on 2001-06-19 for microlens formed of negative photoresist.
This patent grant is currently assigned to Intel Corporation. Invention is credited to Zong-Fu Li.
United States Patent |
6,249,034 |
Li |
June 19, 2001 |
Microlens formed of negative photoresist
Abstract
By forming a microlens of negative photoresist, economical
microlens fabrication processes may be used which, in some
embodiments, may achieve microlenses having good optical clarity
and high thermal stability. In one embodiment, a positive
photoresist may be used as a pattern mask to transfer a pattern to
the negative photoresist. The microlenses may be formed by dry
etching the positive photoresist which acts as a mask to transfer a
pattern to the underlying negative photoresist. At the same time a
scratch protection layer may be formed over regions not overlying
optical sensors.
Inventors: |
Li; Zong-Fu (Gilbert, AZ) |
Assignee: |
Intel Corporation (Santa Clara,
CA)
|
Family
ID: |
23073074 |
Appl.
No.: |
09/280,434 |
Filed: |
March 29, 1999 |
Current U.S.
Class: |
257/432;
430/321 |
Current CPC
Class: |
H01L
27/14621 (20130101); H01L 27/14627 (20130101); H01L
27/14685 (20130101); H01L 31/0216 (20130101); H01L
31/02327 (20130101) |
Current International
Class: |
G02B
1/04 (20060101); G02B 3/00 (20060101); H01L
31/0232 (20060101); H01L 031/023 () |
Field of
Search: |
;257/432 ;430/321,7 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: McPherson; John A.
Attorney, Agent or Firm: Trop, Pruner & Hu, P.C.
Claims
What is claims is:
1. An imaging array comprising:
a photosensitive element;
a passivation layer over said element;
a microlens over said layer; and
a scratch protection layer formed of negative photoresist over said
passivation layer, said scratch protection layer being formed only
in areas where there are no microlenses.
2. The array of claim 1, wherein said negative photoresist is an
epoxy acrylate.
3. The array of claim 2, wherein said epoxy acrylate contains the
fluorene moiety.
Description
BACKGROUND
This invention relates generally to microlens arrays for various
applications including increasing the fill factor of photosensitive
arrays.
Conventional imaging systems may include a light sensing element
such as a charge coupled device (CCD) or a complementary metal
oxide semiconductor (CMOS) sensor. The sensor may include one or
more metal layers and interconnects, interlayer dielectric (ILD), a
passivation layer such as a nitride layer, a color filter array
(CFA), planarization over the CFA and a microlens array over the
CFA. Conventionally, microlens arrays are formed using positive
photoresist materials.
Conventional microlens fabrication involves forming a nitride
passivation, and then opening a bond pad. A polyimide layer is used
to transfer a pattern to the positive photoresist. The polyimide
layer is removed and a color filter array is formed. Thereafter,
the device is planarized. Microlenses are defined in positive
photoresist and then an ultraviolet (UV) bleaching step is used to
improve the transmissivity or optical clarity of the positive
photoresist.
Conventional processes may produce striations. In conventional
processes, the bond pad opening is formed before CFA formation. The
topographic variation due to the surface cavity in the bond pad
areas is a direct source of streaking patterns in CFAs which may
later cause streaking in the resulting images.
Photobleaching may be used with positive photoresist because of the
yellowing that may occur during processing. In addition positive
photoresists are inherently not transparent, even after a hard
bake. This is believed to be due at least in part to the presence
of photoacid compounds in positive photoresist. Thus, the positive
photoresist based microlenses are photobleached using a deep
ultraviolet (UV) source. Even with bleaching, a yellowing problem
may arise upon exposure to heat and humidity.
With existing positive photoresist microlens formation processes,
the bond pads may be left with residues because the bond pad
opening is formed before the microlens is formed. Thus, a final
bond pad area opening may be covered with residuals preventing good
contact to the bond pad.
The thermal stability of positive photoresists is also limited.
When exposed to high temperatures, positive photoresist may change
shape or lose its optical clarity. Thus, it is generally desirable
to avoid high temperatures with positive photoresist based
microlens arrays. However, avoiding high temperatures prevents
using the surface mount process where the silicon chip and
microlens can be heated up to over 200 degrees during the solder
reflow.
Additionally, in conventional processes, the regions adjacent to
the microlenses, which are not situated over photosensitive
elements, are subject to scratching during packaging because the
nitride passivation is completely exposed. This scratching may ruin
the devices and be an undetected source for light
contamination.
Thus, there is a continuing need for improved photosensitive
devices having improved microlens arrays.
SUMMARY
In accordance with one embodiment, a microlens may include a light
collecting element. The element may be formed of negative
photoresist.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the processing of the negative photoresist in
accordance with one embodiment of the present invention;
FIG. 2 is an enlarged cross-sectional view showing a microlens in
the course of fabrication;
FIG. 3 is a cross-sectional view showing the microlens of FIG. 2
after ensuing processing;
FIG. 4 is a cross-sectional view of the microlens structure of FIG.
3 after continued processing;
FIG. 5 is a cross-sectional view of the microlens structure of FIG.
4 after still additional processing;
FIG. 6 is a cross-sectional view of the microlens of FIG. 5 after
completion; and
FIG. 7 is an enlarged cross-sectional view of a photosensitive
device formed in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION
A microlens may be formed from a negative photoresist layer. In one
embodiment of the invention, shown in FIG. 1, a transfer layer such
as a positive photoresist layer may be used to transfer a desired
pattern to the negative photoresist layer. The microlens processing
may be improved and simplified using negative photoresist. In
addition, a scratch protection coating may be formed over the
non-diode array area. The microlenses formed of negative
photoresist may have high thermal stability and transparency in
some embodiments.
Referring to FIG. 1, in one embodiment of the invention, the
process of forming a microlens of negative photoresist begins by
using a conventional nitride passivation step as indicated in block
10. A color filter array (CFA) is then formed (block 12) over the
nitride layer by deposition and patterning steps. Referring to FIG.
2, the nitride layer 24 may cover the bond pad 22. The CFA layer 26
may be deposited and patterned over the nitride passivation 24.
Referring back to FIG. 1, the structure is then planarized (block
14) using negative photoresist. The negative photoresist may be the
same negative photoresist that is conventionally used as the CFA
resist. The CFA resist is typically an acrylic based negative
photoresist. In some embodiments, the acrylic photoresist may be
cross-linked for high thermal stability. For example, some negative
photoresists can withstand temperatures over 200.degree. C. for one
hour without significant degradation in transmissivity. Other
negative photoresists may be utilized as well. The negative
photoresist 28 may be deposited by spin coating. The negative
photoresist is then cured using conventional UV processing
techniques. Referring to FIG. 3, after depositing the negative
photoresist 28, an opening 30 may be formed (block 16, FIG. 1) to
the bond pad using reactive ion etching (RIE)
Returning now to FIG. 1, the pattern of the desired microlens array
may be defined in the positive photoresist transfer layer, as
indicated in block 18. Referring to FIG. 4, the positive
photoresist may be spin coated on the cured negative photoresist
layer and then patterned to create the desired array of microlenses
as indicated at 32. The spin coated photoresist over the bond pad
22 is removed during patterning.
The desired microlens shape may then be transferred to the negative
photoresist planarization layer as indicated in FIG. 1 at block 20
and in FIG. 5. Namely, the positive photoresist in block form,
shown at 32 in FIG. 4, may be melted to form the oval shape
indicated at 34 in FIG. 5. The pattern defined in the positive
photoresist 32 may then be transferred to the negative photoresist
28 through dry etching as shown in FIG. 6. If desired, the dry
etching step may be modified to increase or decrease the curvature
of the resulting microlens 38. In addition, since the previous step
is a dry etching step, for example, using plasma or RIE, it may
automatically clean the bond pad 22. This avoids leaving residues
that would contaminate the bond pads.
In accordance with one embodiment of the present invention, the
completed photosensitive array 42 may include a microlens array 38
as shown in FIG. 7. The CFA 26 may be situated over a nitride
passivation layer 24. The metal layers 46 and 48 may be separated
by the ILD 50. A photosensitive device 54, such as a photodiode, is
situated under each microlens 38.
A negative resist protective coating 40 is formed over the regions
which are not overlying the photosensitive devices 54. The coating
40 may be formed by the same steps that are used to form the
microlens array 38. This provides a scratch prevention layer.
Scratches in this region can adversely effect the performance of
the photosensitive devices.
Use of the negative photoresist process may reduce or eliminates
the need for the polyimide layer. Thus, polyimide ashing removal
may be eliminated in some embodiments.
The CFA striation problem is also reduced or eliminated in some
embodiments of the present invention since the bond pad opening is
not done before CFA formation, in some embodiments. Photobleaching
and the yellowing issue may be avoided since the negative
photoresist does not require bleaching. The negative photoresist
also has high thermal stability and can withstand heating to
temperatures over 200.degree. C. for one hour without significant
degradation in optical properties.
The positive photoresist used for pattern transfer may be a
relatively commonplace photoresist as opposed to special
formulations currently used as microlens resist. A suitable
positive photoresist is AZ4620 from Shipley Company, Marlborough,
Mass. The use of common positive photoresists may result in a cost
saving compared to photoresists used in conventional microlens
formation processes.
In some embodiments, a high stability, high transparency negative
photoresist may be based on an epoxy acrylate resin. Particularly,
epoxy acrylates having the fluorene moiety are particularly
desirable. For example, the negative photoresist known as V259PA
available from Nippon Chemical Company has desirable
characteristics. Its formula is as follows: ##STR1##
A hydroxide may be reacted with anhydride (indicated as a rectangle
above) to form a copolyester. The anhydride block may also contain
a carboxylic group. The base resin may be advantageous over common
acrylic resists because the fluorene pendent groups impart a high
transparency to the microlens. The carboxylic groups may enhance
the base resin solubility in alkaline solutions while producing
good wetting adhesion to substrates. The base resin also exhibits
low volume shrinkage during curing as compared to aliphatic
acrylate.
The epoxy acrylates may have high glass transition temperatures on
the order of 250.degree. C. versus 180.degree. C. for aliphatic
acrylate systems. The epoxy acrylate may exhibit 90 percent light
transmittance for 400-800 nm. after heating to 200.degree. C. for
one hour and then 280.degree. C. for another hour.
While the present invention has been described with respect to a
limited number of embodiments, those skilled in the art will
appreciate numerous modifications and variations therefrom. It is
intended that the appended claims cover all such modifications and
variations that fall within the true spirit and scope of the
present invention.
* * * * *